1. Field of the Invention
The present invention relates to an electronic switching system having a call-forwarding function for effecting call forwarding between subscriber terminals, based on data used for call forwarding stored in a storage unit in the electronic switching system which accommodates a plurality of subscriber terminals, and more particularly, it relates to a system for preventing an endless loop in the call forwarding process.
2. Description of the Related Art
As background art, there are known references as follows:
(a) Japanese Unexamined Patent Publication (Kokai) No. 61-260750 "A Call Forwarding Control System";
(b) Japanese Unexamined Patent Publication (Kokai) No. 54-116809 "A System for Preventing an Endless Loop Call Forwarding in an Incoming Forwarding Call"; and
(c) Japanese Unexamined Patent Publication (Kokai) No. 60-264151 "A System for Registering the Number of an Alternative Destination to Which a Call is to be Forwarded".
The above reference (a) discloses a call transfer between offices in which a further forwarding is inhibited even when another alternative destination is registered. This reference does not disclose the endless loop avoiding technique for avoiding an endless loop which the present invention pertains to.
The above reference (b) discloses a technique for avoiding an endless loop forwarding. The endless loop is prevented, in this reference, by setting a flag during an execution of call forwarding and by checking the flag at each execution of the call forwarding. In contrast, in the present invention, the endless loop is prevented by inhibiting the registration as an alternative destination when an endless loop is anticipated.
The above reference (c) also discloses a technique for preventing an endless loop by calling an alternative destination at a time of registering a call and then registering the call after recognition of the response. If the response cannot be recognized, the registration is inhibited. In this technique, the processes at the time of registering are very complex and hardware is required to recognize the response. In contrast, in the present invention, whether or not an alternative destination is to be registered can easily be checked only by reading discrimination information which is registered before execution of the call forwarding.
As mentioned above, conventionally, there is known an electronic switching system having a call forwarding function which accommodates a plurality of subscriber terminals and includes a storage unit for storing data used to call forward for the subscriber terminals. Based on the data, a call forwarding, e.g., an operation of an intra-office call forwarding, can be executed between the subscriber terminals.
In the above conventional intra-office call forward processing system, a subscriber who wants to accept a call forwarding process conducts a so-called forward registering process (here, an intra-office call forward registering process) by specifying a subscriber terminal as an alternative destination and by registering it in the storage unit. During execution of a call forwarding, a call is forwarded to a terminal which is registered as an alternative destination.
Conventionally, it is not restricted to register one terminal as an alternative destination from a plurality of originating terminals, as later described in more detail with reference to the drawings. Therefore, an endless loop is established in the call forwarding process.
In the conventional art, the endless loop is avoided during the execution of the call forwarding. Therefore, there are problems in that various equipment is used until the process reaches the endless loop which makes the operation of this equipment unnecessary and thus wasteful, and that it is not easy to determine in the executing stage the call forwarding process as an endless loop.